Powders as dosage forms

Powders as dosage forms

Although the use of powders as a dosage form has been replaced largely by the use of tablets and capsules in modern medicine, they represent one of the oldest dosage forms and present certain advantages that have led to their continued use as pharmaceutical dosage forms.

Types of powder dosage forms

Powders as dosage forms can be classified based on their usage and/or phys-ical characteristics as detailed in Sections .

1. Oral powders in unit dose sachets

Powders containing drugs intended for children, such as antibiotics, are commonly made available in powder-filled unit-dose sachets. These pow-ders are intended for administration after premixing with a food product, such a yogurt or juice. For example, the antibiotic Augmentin (amoxicillin in combination with clavulanic acid) and probiotics are available as a sachet. The powder blend is required to have a sweet taste, pleasant fla-vor, appealing color, and an acceptable mouthfeel. Uniform filling of the powder blend in sachets is the only major concern in the dispensing of this dosage form.

2. Powders for oral solution or suspension

Powders for reconstitution into an oral solution or suspension are com-monly dispensed to the patient in multidose bottles. The pharmacist recon-stitutes the powder using water, and the patient is instructed to consume a defined dose, by volume, of the resulting suspension. This mode of drug dispensing is intended to minimize the effects of physical instability of the suspension and/or the chemical instability of the drug compound on storage. This dosage form is exemplified by amoxicillin powder for oral suspension.

The powder blend is required to have a sweet taste, pleasant flavor, appealing color, and an acceptable mouthfeel after reconstitution. Stability of both the dry powder and the reconstituted suspension are important considerations. Also, in addition to the uniform filling of the powder blend in bottles, dose-to-dose uniformity of dispensed solution or suspension after reconstitution of a bottle of powder needs to be established.

3. Bulk powders for oral administration

Herbal medicines, such as laxatives, are commonly dispensed in bulk powder containers for dose dispensing and administration by the patient. The husk of the plant ispaghula as a laxative exemplifies these. These powders must be rel-atively nontoxic with a wide range of well-tolerated doses. These are generally over the counter products that are meant for self-medication by the patient.

4. Effervescent granules

Effervescent granules are sold as bulk powders intended for dispensing of a unit dose and reconstitution with water to form a solution by the patient immediately before administration. Upon contact with water, effervescence is produced by the reaction between an acidic component, such as succinic acid or tartaric acid, and a carbon dioxide-releasing basic component, such as sodium carbonate or bicarbonate. Effervescent granules must be kept in dry state to prevent this reaction before reconstitution by the patient.

5. Dusting powders

Dusting powders are intended for external, local application. The antibiot-ics in powder form for application to open skin wounds exemplify these.

Characteristics of powder blends for their use as dusting powders include low and flexible dose, low and relatively uniform particle size, high density and low aerosolization, and nongrittiness.

6. Dry powder inhalers

Dry powder inhalers (DPIs) are devices that deliver medication to the lungs using an inhalation device in the form of a dry powder. These devices are commonly used for drug delivery for local action, for example, for asthma, bronchitis, and emphysema.

Powder characteristics required for their use in DPIs include good flow, lack of adhesion to the material of package, low and uniform particle size for deposition in the appropriate region of the lung, and an adequate low drug dose.

Advantages of extemporaneous compounding of powders

Compounding of powders for dispensing in pharmacy presents the advan-tages of flexibility in dosing and a relatively good chemical stability. There are, however, disadvantages to extemporaneous compounding of powders as a dosage form. The preparation methods are time consuming and are generally not suitable for drugs that are highly potent, unpleasant tasting, or hygroscopic.

The compounded powders can either be dispensed in unit doses or as bulk powders in a multidose container. The dispensing of bulk powders has a further disadvantage of dosage inaccuracy resulting from several fac-tors such as the BD of powder, consolidation during handling, and the method of measuring the dose by the patient. For these reasons, the dis-pensing of bulk powders is restricted to drugs with some dosage flexibility. These include, for example, herbal and other natural products such as laxa-tives and nutraceuticals and dusting powders intended for external, local application.

Extemporaneous compounding techniques

Extemporaneous compounding of powders as dosage forms in the phar-macy utilizes the same basic pharmaceutical processes, such as weighing, mixing, and sifting—with differences in the equipment used and scale of compounding. For example:

·           Efficient mixing by geometric dilution of the component in the least quantity, such as the potent drug, is carried out by mixing it with an equal quantity of the larger component, such as a diluent, followed by repeated mixing with double the quantity of the larger component.

·           A pestle-and-mortar is typically used for mixing powders in a circular motion with the application of shearing force ( trituration). In addition to mixing, trituration helps reduce the bulkiness, and tends to reduce and normalize the particle size of powder components.

·           Powdered solids can be incorporated into ointments and suspen-sions by forming a paste using water as an insoluble liquid medium. The paste is triturated using a pestle-and-mortar or a spatula on an ointment slab for uniform mixing and particle size reduction (levigation).